I have noticed recently (since it is hotter than heck) an intermittent problem with hot starts on my 1991 300e 3.0L with 80K miles.Cold starts are no problem. If I try to start it when it is hot and has been sitting for 15 to 20 minutes, I hear a small backfire from the intake and it starts a little harder than normal. However, if I try to start it when it is hot immediately after running it , it starts right up. Plugs, distributor cap, injectors, etc. are recent.

Does this happen everytime you do a delayed hot start? Might be some trapped fuel vapor and/or a intake valve that isn't seating properly or fully. Could also be timing, though if it runs and starts OK otherwise, not likely. Bad gas also known to cause this problem.

Jeff,it appears to be on most delayed hot starts (I just noticed it recently). I believe that the timing is OK because it runs smooth and accelerates strong. You mentioned vapor trapped (lock). Is this caused by something or resolved easily. Also, if it is the intake valve seat, is that a big chore.

Have the compression tested. If it a bad valve you will find it that way. Any valve removal or replacement will require removal of the head, so get it to a tech you trust to get proper diagnosis. Trapped fuel would be in the manifold between the intake valve and the air flow sensor. When it gets hot, it vaporizes and can create an explosive situation (so to speak). This can cause damage if not corrected. Could be from a leaky injector(s). The hard start after the backfire is from an intake mainfold full of oxygen depleted atmosphere. Take it to someone you trust and get a good diagnosis.

I tried your first suggestion. Changed to Mobil Supreme Plus gas. The problem appeared to subside after I consumed a quarter tank. Will get compression tested. Thanks.